The present invention relates, in general, to band saw apparatus suitable for cutting relatively hard metal objects and, more particularly, relates to a band saw apparatus and a method for enhancing the cutting rates, the cutting precision and the blade life by minimizing the area of the workpiece engaged by the cutting edge of the blade during cutting.
Various attempts have been made to enhance the cutting rate of a band saw when cutting hard materials. One approach is to form the band saw blade with a height dimension which varies along the length of the blade. Typical of such efforts is the apparatus set forth in U.S. Pat. Nos. 4,205,571, 4,195,543, 4,160,397 and 4,023,448. In these patents an endless band saw blade is formed with a varying height dimension or side elevational profile so that, as the height variations pass over guide means, the blade is urged toward and then permitted to move away from the workpiece.
In most cases such specially formed band saw blades are actually employing a well known principle for improving cutting efficiency, namely, they are formed to reduce the area being engaged by the saw teeth by see-sawing the blade as it is advanced. (See, e.q., U.S. Pat. Nos. 4,160,397, column 2 lines 37-41, 4,205,571, column 2, lines 7-10). In some cases the blade is formed and the guides are set to displace the blade directly toward the workpiece without see-sawing or rocking, but more typically there is angular displacement.
When hard metallic workpieces are being cut, the saw blade teeth tend to be displaced into the workpiece only to a small degree before there is a reaction force which tends to move the saw arm (or drive wheel support frame) away from the workpiece. Thus, after some initial set or small penetration of the teeth into the work, the remainder of the displacement resulting from blade height variations tends to produce upward movement of the saw arm away from the workpiece. Since band saws typically include a pneumatic or hydraulic feed cylinder and control circuits and associated feed linkage which control the rate at which the saw is advanced through the workpiece, the feed cylinder and control circuit act as a constraint against free movement of the band saw.
In some saws the feed cylinder and control circuitry will not let the arm back rapidly away from the workpiece to accomodate variations in blade height which are greater than the initial set of the teeth into the hard metallic workpiece. This will cause the saw blade to bend and to be stressed. In other saws, the feed assembly will allow the saw to move away from the workpiece as the blade height increases, but will not allow a rapid return toward the workpiece as the valleys in the blade are passed over the blade guides. If the return toward the workpiece which is induced by the valleys in the blade height is at rate which is greater than the feed rate, the feed cylinder and control circuit retard the arm movement to the saw arm back toward the workpiece feed rate. Such retarding or slowing of the saw arm movement results in a dramatic reduction in cutting force while the saw arm tries to catch up to the blade displacement. In the extreme the force will be zero and stressful bouncing and bending of the blade will occur. This cutting force reduction reduces cutting efficiency during part of the cutting process, although the overall efficiency is improved as a result of the improvement during the portion of the cut in which the blade is being urged toward the workpiece and the arm is rebounding away from the workpiece.
Although not specifically recognized in the prior art, the effect of the feed cylinder constraint has heretofore been accommodated in one of two ways. First, the height variation along the blade can be selected so that it is not substantially greater than the depth to which the teeth can be initially set or driven into the work. The disadvantage of this "solution" is that it does not maximize cutting efficiency. A second approach is to spread the blade guides from the workpiece sufficiently to enable bending of the saw blade along the longitudinal axis of the blade to accomodate restraint in either the movement toward or away from the workpiece. This results in bowing of the blade and produces angular variations which reduce the see-sawing effect. This "solution," however, is very disadvantageous in that it can fatigue the blade, and it will cause wandering of the blade as it cuts through the work. The amount of deviation of the saw blade from a straight line cut is proportional to the third power of the separation of the guides from each other.
If the blade guides are set next to the workpiece so that blade bending is insignificant, and if the height variation is too great, feed cylinder constraint will cause the teeth to dig into the workpiece to a degree which will either stop the saw or shear off the teeth from the blade. Since this possibility is unacceptable, specially formed prior art blades either have had limited height variations or have been used with blade guides which are spaced from the workpiece, or compromises of both approaches.
Ideally, however, enhance cutting efficiency is obtained when the teeth are urged into the workpiece beyond the initial set to just short of digging in to a point which would stop the saw or shear the teeth. For maximum efficiency in the cutting of a hard metallic material, the teeth should be forced into the material until they start to dig in so they will cut and pull a chip and then the teeth should be released before shearing or stalling of the saw. Moreover, the cutting force should be applied and maintained as a substantially constant force, below the force which will cause the teeth to dig in, for as long as possible during the entire blade displacement cycle. Additionally, and most preferably, the area being cut at any time should be minimized by see-sawing or rocking of the saw blade so as to increase the pressure per unit area of the teeth and ease penetration of the material being cut without having to increase the total force on the blade.
Specially formed saw blades have other disadvantages. It is inherent that in a sinusoidal, stepped or otherwise specially formed blade the areas of the blade which project outwardly toward the workpiece the farthest will undergo the greatest amount of stress and wear. Thus, over the length of a band saw blade, all of the downwardly projecting areas will tend to wear first and to have the greatest potential for blade fatiguing stress risers.
It is possible to change specially formed blades to vary the amplitude of blade displacement and thus, to some degree, the see-saw cutting action and pressure applied, and it also is possible to vary the cutting speed to change the frequency of pressure variation. These changes, however, are not easily or quickly accomplished. For band saw apparatus in which the production runs are short and the materials being cut are changed frequently, for example, in a job-shop, repeated blade changing can be a substantial disadvantage.
Another approach to achieving a see-saw cutting action and thereby varying the pressure applied by a band saw blade during cutting is disclosed in U.S. Pat. No. 4,127,045. In this patent a pivotal arm band saw is shown which has a blade guiding assembly that is formed to displace the saw blade toward and permit movement away from the workpiece by a stepcam and follower.
The cutting dynamics for the apparatus of U.S. Pat. No. 4,127,045 are the same as above described for specially formed blades. As the cam pushes the blade toward the workpiece and saw table, the blade flexes between the blade guides and the workpiece because the arm feed cylinder resists reaction displacement of the arm upwardly. The guides in U.S. Pat. No. 4,127,045 are widely spaced from the workpiece which will permit blade flexure. Displacement of the blade toward the work table by one guide does change the angle of engagement of the workpiece with the work to enable a see-saw cutting action, but the wide setting of the guides and the constraint of the cutting arm by the feed cylinder inhibit the effectiveness of such see-saw action and the depth to which the teeth are driven into the workpiece.
Generally it is well known in the cutting art, and particularly in connection with power hack saws and power grinders, to vary the angle of cutting during the cut. MARVEL brand hack saws, produced by Armstrong-Blum Manufacturing Co. of Chicago, Ill. for example, have for many years used a "Roll Stroke" blade motion which progressively raises one end of the saw blade while lowering the other as the blade is drawn through the work. This action is repeated many times over the full cut of the workpiece. Grinders similarly have been constructed in which the pivotal arm upon which the abrasive wheel is mounted is itself eccentrically mounted to displace the pivot point of the arm and thereby vary the area being cut. It also is common machining practice to rotate or roll the workpiece as it is cut to reduce the area engaged at any time by the cutting tool, and all of these methods maintain higher unit cutting forces by reducing the cutting area.
In the same manner as see-sawing reduces the area of a workpiece being engaged by the saw blade, the shape of the workpiece, and particularly the width dimension, can effect the cutting rate substantially. A piece of round stock, for example, initially presents a relatively small area (length along the blade) to be cut. As the blade cuts through the cylinder, the area increases until the center of the workpiece is reached. The area of the workpiece along the blade which is being cut then diminishes as the blade moves down to the bottom of the workpiece.
The effect of the changing area of engagement of the saw blade with the workpiece as a function of workpiece shape on the efficiency of cutting workpieces has not been considered in any detail. While see-sawing the blade and/or using specially formed stepped or wavy blades enhances efficiency, the effeciency of cutting a workpiece can be further enhanced by considering workpiece width at various times during the cut and making adjustments to saw performance characteristics such as feed rate, blade oscillation, cutting pressure and/or blade speed. Moreover, such variations also can be used to enhance cutting effeciency in connection with conventional non-oscillating or vibrating saws.